Hydrocarbon pipelines constructed from carbon steel are vulnerable to corrosion, especially corrosion caused by organic acids and acid compounds of H2S and CO2 dissolved in water. To prevent or avoid material loss from corrosion, it is common to inject corrosion inhibitors into the pipeline. However, the entire pipeline, including the very top of the line at the twelve o′clock position, is vulnerable to corrosion since the corrosion inhibitors tend to stay with the liquid at the bottom of the line. The gases at the top of the line include hydrocarbon gases, water vapor and the acidic components described above. Corrosion rates for untreated pipelines through the wall of the pipe may exceed 10 millimeters per year so corrosion mitigation is critical to economic and safe pipeline operation.
Any pipeline with stratified flow is particularly vulnerable to corrosion as the corrosion inhibitors typically remain concentrated in the liquid solution at the lower portion of the pipeline. Most hydrocarbon lines have stratified flow where liquids are at the bottom and gases are in the upper portions.
One technique for applying corrosion inhibitors is batch inhibition which consists of to periodically sending a device called a “pig” that approximately corresponds with the interior dimension of the pipe and includes circumferential grooves and other surface structures to apply the corrosion inhibitor chemicals to the full interior surface of the pipe. Unfortunately, batch inhibition using pigging can be expensive and many pipelines are designed or constructed in a manner that cannot be pigged. To the extent that a pipeline may have been designed to be amenable to pigging, a pig disrupts production through the pipeline, especially as it first enters the pipeline. In some circumstances, downstream processes are dependent upon continuous flow through the pipeline. For example, liquefied natural gas (“LNG”) liquefaction plants are typically located near the shore where the liquefied natural gas may be loaded onto a LNG ship. The liquefaction plant may be remote and therefore will depend on a continuous flow of natural gas from the pipeline that carries the natural gas from the field. A potential interruption of the gas flow would create considerable costs as the plant is shut and restarted.
Pigs made of gelatin are an interesting alternative for pipelines that have tight turns or narrow passages along the length. However, gelatin materials are not suitable for warmer pipelines and do not necessarily assume the full diameter of the pipeline after a passing through a substantially smaller diameter valve or other restriction.
Corrosion resistant alloy materials have been used in hydrocarbon production facilities and pipe for years and will be effective in avoiding the dramatic corrosion rates that this invention is seeking to control. However, such corrosion resistant alloys are far more expensive than common carbon steel and are typically used in areas where it is impractical to apply corrosion inhibition.